Most neutralizing antibodies to the envelope (env) glycoprotein of human immunodeficiency virus type 1 (HIV-1) exhibit limited cross-reactivity with other HIV-1 isolates. It is becoming increasingly clear, however, that much more broadly neutralizing antibodies to conformation dependent antigenic epitopes can be developed. Ideally, an effective vaccine should elicit potent, broadly neutralizing antibodies to a number of epitopes. Thus, it is important to understand the determinants in the env glycoprotein which lead to the production of antibodies which are broadly reactive. Like other viral membrane proteins, HIV-1 env forms an oligomeric complex, specifically a noncovalently associated homodimer. To assess the effects of env quaternary structure on its antigenic potential, we raised nearly 200 monoclonal antibodies (MAbs) against native, soluble, oligomeric env. We found that nearly 70% of the MAbs recognized conformationally sensitive epitopes, and that two-thirds recognized divergent HIV-1 strains. In addition, we identified broadly neutralizing MAbs which recognize oligomer specific epitopes in the gp41 ectodomain as well as MAbs sensitive to, but not strictly dependent upon, env quaternary structure. The antigenic structure of the gp41 ectodomain is particularly sensitive to quaternary interactions. Thus, the antigenic structure of HIV-1 env is strongly influenced by its oligomeric structure, and by immunizing with soluble, native, oligomeric env glycoprotein, a new class of neutralizing antibodies, as well as many antibodies whose reactivities are completely or partially dependent on env quaternary structure, are obtained. The overall goal of this project is to more fully evaluate the consequences of env oligomerization on its antigenic structure. Specifically, we will: 1) Identify regions of the HIV-1 env glycoprotein where antigenic structure is strongly influenced by quaternary interactions; 2) Identify highly conserved epitopes; 3) Identify neutralizing antibodies (particularly MAbs to novel epitopes, such as oligomer specific and oligomer sensitive MAbs); 4) Determine if oligomer dependent or sensitive antibodies comprise a significant fraction of total neutralizing activity in HIV-1 positive human serum; and 5) Compare directly the immunogenicity of native dimeric and monomeric env.